Filled Skutterudites for Advanced Thermoelectric Applications

ABSTRACT

A low-cost filled skutterudite for advanced thermoelectric applications is disclosed. The filled skutterudite uses the relatively low-cost mischmetal, either alone or in addition to rare earth elements, as a starting material for guest or filler atoms.

This application is a divisional of application Ser. No. 10/979,005,filed Nov. 1, 2004.

TECHNICAL FIELD

The present invention relates to thermoelectric devices which utilize athermal gradient to generate electrical power and can be used in heatingand cooling applications. More particularly, the present inventionrelates to filled skutterudites as thermoelectric materials, thefabrication of which includes use of the low-cost mischmetal alloys andmischmetal-transition metal alloys as starting materials.

BACKGROUND OF THE INVENTION

Due to an increasing awareness of global energy needs and environmentalpollution in recent years, much interest has been devoted to thedevelopment and use of thermoelectric (TE) materials for automotive andother applications. TE devices are capable of transforming heat directlyinto electrical energy and also acting as solid state coolers. Throughtheir energy-generating capability, TE devices are capable of enhancingthe ability of internal combustion engines to convert fuel into usefulpower. The cooling capability of TE devices can contribute to aresolution of the greenhouse concerns associated with refrigerant use,as well as enable new design concepts for heating and air conditioningand improve the reliability of batteries. TE-based waste heat recoveryis also applicable to modes of transportation such as diesel-electriclocomotives, locomotive diesel engines, automotive diesel engines,diesel-electric hybrid buses, fuel cells, etc.

The energy conversion efficiency and cooling coefficient of performance(COP) of a TE device are determined by the dimensionless figure ofmerit, ZT, defined as ZT=S² T/ρκ_(total)=S²T/ρ(κ_(L)+κ_(e)), where S, T,ρ, κ_(total), κ_(L), and κ_(e) are the Seebeck coefficient, absolutetemperature, electrical resistivity, total thermal conductivity, latticethermal conductivity and electronic thermal conductivity, respectively.The larger the ZT values, the higher the efficiency or the Coefficientof Performance (COP). An effective thermoelectric material shouldpossess a large Seebeck coefficient, a low electrical resistivity and alow total thermal conductivity.

Binary skutterudites are semiconductors with small band gaps of 18 100meV, high carrier mobilities, and modest Seebeck coefficients. Binaryskutterudite compounds crystallize in a body-centered-cubic structurewith space group Im3 and have the form MX₃, where M is Co, Rh or Ir andX is P, As or Sb. Despite their excellent electronic properties, binaryskutterudites have thermal conductivities that are excessively high tocompete with state-of-the-art thermoelectric materials. It was foundthat filled skutterudites have much lower thermal conductivities.Therefore, filled skutterudites are increasingly popular as athermoelectric material due to their lower thermal conductivities.

Filled skutterudites can be formed by inserting rare earth guest atomsinterstitially into large voids in the crystal structure of binaryskutterudites. The chemical composition for filled skutterudites can beexpressed as G_(y)M₄X₁₂, where G represents a guest atom, typically arare earth atom, and y is its filling fraction. Compared to binaryskutterudites, the lattice thermal conductivities of the rare earthfilled skutterudites are significantly reduced over a wide temperaturerange. This property of filled skutterudites is due to the scattering ofheat-carrying low-frequency phonons by the heavy rare earth atoms, whichrattle inside the interstitial voids in the skutterudite crystalstructure.

In recent years, both n- and p-type rare earth filled skutterudites havebeen reported to have superior thermoelectric figure of merit (ZT)values in excess of 1 for temperatures above ˜500 degrees C. For rareearth filled skutterudites, the best n-type materials are La—Fe—Co—Sband Ce—Fe—Co—Sb skutterudites. The best p-type materials are Yb—Co—Sband Ba—Ni—Co—Sb. FIG. 1 shows ZT values of recently-discovered filledskutterudites as compared to those of state-of-the-art thermoelectricmaterials.

The relatively high cost of high-purity starting materials for rareearth filled skutterudites contributes to the overall cost of thefabricated thermoelectric devices. Therefore, filled skutterudites areneeded which utilize low-cost starting materials to decrease the overallcost of thermoelectric devices.

SUMMARY OF THE INVENTION

The present invention is generally directed to filled skutteruditeswhich are low-cost and suitable for use as a thermoelectric material.According to the invention, mischmetal (Mm), in addition to a transitionmetal alloy or both rare earth and transition metal alloys, is used as astarting material for the fabrication of both n-type and p-type filledskutterudites. Mischmetal is an alloy of both Ce (˜50 wt. %) and La (˜50wt. %). In a typical embodiment, the filled skutterudite has acomposition of Mm_(y)Co₄Sb₁₂(0<y≦1). Use of mischmetal as a startingmaterial for fabrication of the skutterudite provides a low-costalternative to high-purity rare earth starting materials whichcharacterize conventional skutterudite fabrication processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a line graph which compares the thermoelectric figure of merit(ZT) for recently-discovered filled skutterudites with thethermoelectric figure of merit for state-of-the-art thermoelectricmaterials;

FIG. 2 illustrates a body-centered cubic crystal structure of a filledskutterudite fabricated according to the present invention; and

FIG. 3 is a flow diagram which illustrates sequential process stepscarried out in a typical method of fabricating a filled skutteruditeaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, a portion of a filled skutterudite according to thepresent invention is generally indicated by reference numeral 10. Thefilled skutterudite 10 is shown in the form of a body-centered cubiccrystal structure having the composition G_(y)M₄X₁₂, where G representsguest (filling) atoms 12; y is the filling fraction of the guest atoms12; M represents transition metal atoms 14; and X represents atoms fromgroups IVA-VIA of the periodic table 16. In the filled skutterudite 10,a transition metal atom 14 is enclosed in each X₆ tetrahedron formed bythe X atoms 16. The guest atoms 12 are enclosed in the irregulardodecahedral cages formed by the adjacent tetrahedra of X atoms 16.

In the filled skutterudite 10, the X atom sites 16 may be C, Si, Ge, Sn,Pb, N, P, As, Sb, Bi, O, S, Se, Te or Po, or combinations of theseatoms. Preferably, the X atoms 16 are P, As or Sb. Most preferably, theX atoms 16 are Sb.

The transition metal atoms 14 may be Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir,Ni, Pd, Pt, Cu, Ag or Au, or combinations of these atoms. Preferably,the transition metal atoms 14 are Co, Rh or Ir. Most preferably, thetransition metal atoms 14 are Co.

The guest atoms 12 in the filled skutterudite 10 may be rare earthatoms, Na, K, Ca, Sr, Ba, and combinations of these atoms. According tothe present invention, a source or starting material of the guest atoms12 is mischmetal (Mm) alloy, which is an alloy of mostly Ce (about 50wt. %) and La (about 50 wt. %). The mischmetal may be used alone or incombination with a rare earth metal or metals, or with Na, K, Ca, Sr,Ba, and combinations of these metals, as the source or starting materialfor the guest atoms 12. These rare-earth metals include those from thelanthanide series, such as Ce, Pr, Nd, Sm, Eu and Gd, as well as thosefrom the actinides series, such as Th and U. In a typical embodiment,the composition of the filled skutterudite 10 is Mm_(y)Co₄Sb₁₂ (0<y≦1).

In the filled skutterudite 10, “rattling” of the guest atoms 12 in theirregular dodecahedral cages formed by the adjacent tetrahedra of Xatoms 16 reduces the lattice thermal conductivity of the material whileminimally affecting carrier mobility by scattering phonons. Use ofmischmetal as a source or starting material for the guest atoms 12reduces the overall cost of the filled skutterudite 10, since mischmetalis relatively low in cost compared to high-purity rare earth elementswhich serve as the starting material for the filler or guest atoms inconventional filled skutterudites.

An illustrative method of fabricating the filled skutterudite 10according to the present invention is shown in the flow diagram of FIG.3. In step 1, mischmetal, transition metal and X powders are provided.These powders are available from commercial vendors. In step 2, aprecursor pellet is prepared using the mischmetal and transition metalpowders, or the mischmetal and other guest element powders incombination with the transition metal powder, using the properstoichiometric ratios. This step may be carried out by using aninduction furnace process or any other process which is capable ofproducing high temperatures (typically>1,200 degrees C.) followed byrapid cooling or quenching. In step 3, the precursor pellet formed instep 2 is mixed with the X powder. In step 4, the mixture containing theprecursor pellet and X powder is sintered. This is followed by annealingof the mixture (step 5) at a temperature of typically about 500˜1000degrees C. for at least about 24 hours. Finally, the annealed mixture ishot-pressed into the filled skutterudite material at a pressure oftypically at least about 57,200 psi.

While the preferred embodiments of the invention have been describedabove, it will be recognized and understood that various modificationscan be made in the invention and the appended claims are intended tocover all such modifications which may fall within the spirit and scopeof the invention.

1. A filled skutterudite comprising a chemical composition ofG_(y)M₄X₁₂, where G comprises mischmetal as a source of guest atoms;wherein mischmetal comprises at least two rare earth elements and one ormore non-rare earth impurities, y is a filling fraction of said guestatoms, M represents transition metal atoms, and X represents atoms fromgroups IVA-VIA of the periodic table.
 2. The filled skutterudite ofclaim 1 wherein said chemical composition is Mm_(y)Co₄Sb₁₂ (0<y≦1),where Mm is mischmetal.
 3. The filled skutterudite of claim 1 wherein Mis a transition metal selected from the group consisting of Mn, Tc, Re,Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au.
 4. The filledskutterudite of claim 3 wherein M is Co, Rh or Ir.
 5. The filledskutterudite of claim 4 wherein M is Co.
 6. The filled skutterudite ofclaim 1 wherein X is an atom selected from the group consisting of C,Si, Ge, Sn, Pb, N, P, As, Sb, Bi, O, S, Se, Te and Po.
 7. The filledskutterudite of claim 6 wherein X is P, As or Sb.
 8. The filledskutterudite of claim 7 wherein X is Sb.
 9. A filled skutteruditecomprising a chemical composition of G_(y)M₄X₁₂, where G representsmischmetal alone or in combination with at least one rare earth element,as a source of guest atoms; wherein mischmetal comprises at least tworare earth elements and one or more non-rare earth impurities, y is afilling fraction of said guest atoms, M represents transition metalatoms, and X represents atoms from groups IVA-VIA of the periodic table.10. The filled skutterudite of claim 9 wherein said rare earth elementis a rare earth element selected from the group consisting of Ce, Pr,Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and combinations of these atoms.11. The filled skutterudite of claim 9 wherein M is a transition metalselected from the group consisting of Mn, Tc, Re, Fe, Ru, Os, Co, Rh,Ir, Ni, Pd, Pt, Cu, Ag and Au.
 12. The filled skutterudite of claim 11wherein M is Co, Rh or Ir.
 13. The filled skutterudite of claim 12wherein M is Co.
 14. The filled skutterudite of claim 9 wherein X is anatom selected from the group consisting of C, Si, Ge, Sn, Pb, N, P, As,Sb, Bi, O, S, Se, Te and Po.
 15. The filled skutterudite of claim 14wherein X is P, As or Sb.
 16. The filled skutterudite of claim 15wherein X is Sb.